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Doublet or Triplet?

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#26 jrbarnett

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Posted 09 December 2012 - 02:14 PM

I guess he was "lucky" in his uniformed analysis. Having looked through several NP-101s as well as TV-101s and Gensises, I think he nailed it.

Just because someone can drive a car, doesn't mean they know squat about its capabilities. Take my mom, for example. She drives a Porsche Cayenne S. Now she couldn't tell you the first thing about its cornering limits, top speed, or how it stacks up in this regard to other vehicles. I've never driven her Porsche, but without ever having driven it, I do know more about its capabilities (as well as how to better utilize them) than she does. Were she to post on a performance automobile forum making claims about her Porsche's capabilities, she'd actually be the one trolling as she hasn't a real clue.

The moral of the story is this: use of an item, telescope, automobile or anything else, should not automatically be equated to being knowledgeable about the item. Likewise, the suggestion that someone who has not used that item cannot, nonetheless, have valid knowledge about it is preposterous. The scientists and engineers that built the space shuttle never flew it, but you can be darned sure that they know much more about the shuttle than any of the pilots. :winky:

Regards,

Jim
 

#27 bratislav

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Posted 09 December 2012 - 06:30 PM

As far as doublets being the best.. they have significant field curvature and less than perfect color correction though in the smaller sizes/slower focal ratios it may not be apparent. They may be the best compromise for certain observers, but triplets and Petzvals can provide more perfect views.


Actually, doublets, triplets, quadruplets and even quintuplets will all have the same field curvature (approx 35% of focal length), as long as all of those lenses are bunched up front.

The only way to flatten the field is to have another lens group widely spaced, closer to focus.
That then becomes a near focal plane field flattener (the way AP and TEC do it), an integrated positive lens group (Petzval, the way TeleVue does it), or negative lens group (as in Takahashi FSQ variants as well as some older Pentax designs).
The key is that second lens group is much closer to focus.

Besides, color correction of doublets is not actually the problem. Triplet does not give more room to a designer to improve color that much (it does, but comparatively very little; residual color is directly proportional to difference in partial dispersion, so given ED and main mating element(s), residual longitudinal color aberration will be very similar in an objective with 2 and 3 elements).
What triplet DOES buy you is the ability to distribute power, and make radii much more shallow compared to a doublet. This results in vastly reduced spherochromatism.
(triplet can also allow the designer to push the correction farther into violet/UV - insignificant for visual, could be important for CCD work)
 

#28 la200o

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Posted 09 December 2012 - 07:24 PM

I guess he was "lucky" in his uniformed analysis. Having looked through several NP-101s as well as TV-101s and Gensises, I think he nailed it.

Just because someone can drive a car, doesn't mean they know squat about its capabilities. Take my mom, for example. She drives a Porsche Cayenne S. Now she couldn't tell you the first thing about its cornering limits, top speed, or how it stacks up in this regard to other vehicles. I've never driven her Porsche, but without ever having driven it, I do know more about its capabilities (as well as how to better utilize them) than she does. Were she to post on a performance automobile forum making claims about her Porsche's capabilities, she'd actually be the one trolling as she hasn't a real clue.

The moral of the story is this: use of an item, telescope, automobile or anything else, should not automatically be equated to being knowledgeable about the item. Likewise, the suggestion that someone who has not used that item cannot, nonetheless, have valid knowledge about it is preposterous. The scientists and engineers that built the space shuttle never flew it, but you can be darned sure that they know much more about the shuttle than any of the pilots. :winky:

Regards,

Jim


l entirely disagree. These labored analogies are preposterous.

Regards,
Bill
 

#29 KWB

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Posted 09 December 2012 - 08:37 PM

Time to let this thread simmer down a bit so it's being locked for the time being subject to another moderator review.
 






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